<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        Array.prototype.myJoin = function(callback){
            var result = ""
            for(var i =0;i<this.length-1;i++){
                result += this[i].toString()+callback
            }
                result += this[i].toString()
            return result;
        }

        arr = [100,200,300]
        console.log(
            arr.myJoin('*')
        )
    </script>
</body>
</html>